Negative feedback amplifier



g- 1939. F. B. ANDERSON El AL 2,170,045

NEGATIVE FEEDBACK AMPLIFIER Filed April 15, 1938 FIG.

2 Sheets-Sheet 1 PILOT WIRE GAIN REGULATOR FIGS cz /v EBANDERSONINVENTORS AJKCLEMENT By /.6.W/L'SON 6- C CL ATTORNEY Aug. 22, 1939. F.B. ANDERSON E1 AL NEGATIVE FEEDBAC K AMPLIFIER 2 Sheets-Sheet 2 FiledApril 15, 1958 PLATE 0F LAST T085 ll-II namvosksolv By I.G.W/LSONATTORNEY vvvvvvvv INVENTOR$LWCLEMENT GRID 0F PLATE 0! FIRS T TUBE L457TUBE Llllllllllll. llll.

UNITED STATES NEGATIVE FEEDBACK AIWPLIFIER Frithiof B. Anderson,Elberon, and Andrew W. Clement, Summit, N. J., and Ira G. Wilson, NewYork, N. Y., assignors to Bell Telephone Laboratories, Incorporated, NewYork, N. Y., a corporation of New York Application April 15, 1938,Serial No. 202,180

' 13 Claims. (01. 179-171) proving operation of the system for instance,mission from the tube generator to the feedback amplifiers employinggain reducing feedback for path dependent upon the impedance of thebridge reducing modulation in the amplifier, increasing arm thatincludes the output transformer, notthe gain stability of the amplifier,or controlling withstanding the fact that feedback may balance the gainor the gain-frequency characteristic of the bridge to a driving voltageoriginating in the the amplifier, load. Consequently, departure of theimpedance plifier and the circuit a network affording desirfar above theutilized frequency range or oper 20 able loss and impedancecharacteristics for transating frequency range of the amplifier.Moremission between the amplifier and the circuit, yet over, the effectof such departure on the feedback giving the impedance faced by theamplifier a may be greatly increased by interaction of thecharacteristicthat avoids menace to the singing impedance of thetransformer with the impedmargin. ances in other arms of the bridge. Forexample, 25

In one specific aspect the invention is applied the bridge may be anequal e bridge of the yp to a negative feedback vacuum tube amplifier ofdisclosed in A. L. Stillwell Patent 1,933,758, March the type that hasan output bridge connecting 1935, in the above-mentioned 65 0f the lasttube of the amplifier to the amplifier H. S. Black Patent 2,102,671, andresonance may output transformer and to the feedback path. occur betweena reactance introduced by the 30 That type is disclosed, for example, inFig. 65 transformer and a reactance of one of the two of H. S. BlackPatent 2,102,671, December 21, inverse impedance arms used to effectequaliza- 1937. tion; and such resonance may further unbalance If,without feedback, the bridge arm containthe bridge and may produceviolent changes in 'ing the output transformer or load is conjugate thefrequency characteristic of the impedance 35 to the feedback path, thetransformer has no attached to the plate and cathode of the tube; effecton the transmission from the tube to the and, as explained hereinafter,such further unfeedback path, excepting eifects of stray capacibalanceand such changes in frequency charties to ground. In other words,excepting the acteristic may increase the effect of the output 40 lattereffects, with passive balance of the bridge transformer in changing thegain and phase of 40 the feedback is independent of the transformer.transmission around the feedback loop and re- However, the bridge isoften purposely given pasducing singing margin of the amplifier,especially sive unbalance. As brought out, for instance, in where theplate impedance in the tube is high,

H. S. Black Patent 2,131,365, September 27, 1938, as in the case of asuppressor grid pentode. the purpose of giving the bridge the passiveun- In accordance with a feature of the invention, 5 balance may be, forexample, to reduce reflection the impedance'of the output transformer isbuilt to the load while working the tube into its optiout to a constantresistance for a range of fremum impedance from the standpoint ofmoduquencies including a wide range extending uplation. As thereexplained, the feedback can so wardly from the transmission frequencyrange change the amplifier output impedance as to proof the amplifier,without unduly disturbing the duce a match between the amplifieroutputimtransmission in the latter range. This avoids pedance and the loadimpedance while the tube undesirable efiects of the transformer on theworks into its optimum value of load impedance, operation of theamplifier, reducing the effect of and this optimum value may be fardifferent the transformer in lowering the singing margin.

A further feature of the invention is that the This invention relates towave translating systems, as for example, systems for amplifyingelectric waves, and relates especially to such systems employinggain-reducing feedback for im- An object of the invention is to controlfeedback in such systems, for example, for reducing singing tendency ofsuch amplifiers or increasing their margin against singing.

In accordance with the invention, where a negative feedback amplifierhas a load circuit or attached circuit whose impedance outside thetransmission band of the amplifier menaces the singing margin, there isassociated with the amfrom the actual plate impedance in the tube.

For example, it may be a fifth or a tenth of the plate impedance in thecase of a high impedance tube such as a pentode.

The passive unbalance may render the transof the transformer from itsnominal resistance value may tend to affect the transmission from thetube generator to the feedback path, or in other words, may tend toaffect the feedback or the propagation around the feedback loop, and 15as a result, the singing margin of the amplifier may be reduced. Suchdeparture may occur at frequencies outside of the transmission band ofthe transformer, asfor example, at frequencies means used for buildingout the transformer impedance to a constant resistance is an appropriateimpedance, resistive at frequencies well above the transmission band,connected in series between the transformer and the plate of the outputtube. As explained hereinafter, this precludes objectionable resonanceof the building out means with the capacity to ground in the transformerat these high frequencies and, moreover, is especially advantageous inthat it removes the capacity to ground of the output coil from the plateof the tube and substitutes for that capacity a much smaller capacity.

Other objects and aspects of the invention will be apparent from thefollowing description and claims.

Fig. 1 of the drawings is a schematic circuit diagram of an amplifierembodying the specific form of the invention referred to above; and

Figs. 2 to 5 show circuit diagrams facilitating explanat on of theinvention.

The amplifier of Fig. 1 is generally similar to that of Fig. 65 of theabove-mentioned Patent 2,102,671, and is suitable, for example, as aline amplifier or repeater amplifier for amplifying carrier frequenciesin the range from 12 kilocycles to 60 kilocycles transmitted over a nneteen-gauge non-loaded cable c rcuit comprising the incoming line I andthe outgoing line I.

Condensers 2 and 2 in the input circuit and condenser 3 in the outputcircuit serve to make the amplifier impedances more nearly match thecable impedance, and also to make the line circuits of the amplifier anopen circuit to direct current used, for example, for cable testing.Moreover, these condensers contribute loss for frequencies below the12-60 kilocycle band, to facilitate meeting the requirement that theamplifier again be considerably less than the cable attenuation at theselow frequencies for all gain settings of the amplifier. Further, thecondensers 2 and 2 have equal capacities and have their junctiongrounded, to increase longitudinal balance of the system while helpingto meet the requirement of impedance matching and the requirement ofattenuation outside the transmission band.

The amplifier comprises suppressor grid pentodes V1, V2 and P. The tubesare connected in cascade arrangement by interstage networks 4 and 5,which may be, for example, networks generally similar to the interstagenetworks of F'gs. 15 and 16 of the copending application of H. S. Black,Serial No. 114,390, filed December 5, 1936, for Wave translationsystems. The amplifier comprises an input transformer if, an outputtransformer T, a negative feedback path F, and output bridge B. Thebridge connects the plate circuit of tube P to the feedback path and theoutput transformer in the general manner disclosed, for example, in Fig.65 of the Patent 2,102,671 or in the above-mentioned Stillwell patent.The bridge serves as a transmission equalizer of the constant resistancetype in the general manner explained in those patents, especially theStillwell patent. The feedback path is in one arm of the bridge. Theoutput transformer is in another arm. Of the remaining four arms, onecontans the plate resistance RP of tube P, a second contains resistanceR, and the other two are inverse impedances Zn and Z21.

The feedback through path F reduces the amplifier gain, but gives greatimprovement in quality and stability and greatly reduces harmonicdistortion. The feedback path comprises a 1.. gain control potentiometer1 including a poten tiometer resistance 8 having tap conductors 6, l0and H.

Path F also comprises a condenser potentiometer of the type disclosed inthe application of F. B. Anderson et al., Serial No. 44,050, filedOctober 8, 1935, for Voltage or gain control, issued as Patent2,106,336, Jan. 25, 1938. This condenser potentiometer is formed of afiat gain control condenser G0, a flat gain regulator condenser GR, andtwo trimmer condensers TR and t The flat gain control condenser GCserves for varying the, amount of feedback to adjust the flat gain ofthe amplifier through a range of 10 decibels, for example. The fiat gainregulator condenser GR may be automatically operated, for example, by apilot wire gain regulator system l5 such as that disclosed in F. A.Brooks Patent 2,075,775, April 6, 1937, for varying the amount offeedback in order to compensate for the fiat portion of the variationsin the line loss with line temperature.

The potentiometer I comprises a condenser l2, an inductance I 3 and astopping condenser M. Condenser I2 and inductance l3 improve the highfrequency phase margin of the amplifier against singing, for the.potentiometer settings on taps 9 and I0; and the inductance l3 improvesthe high frequency phase margin for the setting on tap H. The tappedresistance 3 supplies uniform losses over the 12-60 kilocycle band inthe feedback loop, the capacity l2 and inductance i3 exerting littleeffect in that band; the capacity l2 contributes phase margin at highfrequencies above that band; and the inductance 13 in the shunt arm ofthe potentiometer neutralizes some of the capacity bridging this arm andfurther improves the phase shift.

Across the secondary winding of the input transformer is a terminatingresistance 16 having a tap 11 that divides the resistance into twoportions 18 and E9. The resistance l8 connects tap l1 and the highvoltage terminal 20 of the winding. The resistance l9 connects the tap I7 and the low voltage terminal 2| of the transformer. A movable contact22 connected to the grid of tube V1 can be shifted from terminal 20 totap I? for reducing the flat gain of the amplifier. A condenser 23shunts resistance It; and a condenser 24 shunts resistance 19. Thecondensers 23 and 24 and the distributed capacity inherent in thesecondary winding reduce the unfavorable high frequency phase shiftthat, especially with contact 22 on tap 11, the

input transformer and its terminations introduce in transmission aroundthe feedback loop, and thus increase the singing margin of the amplifieror its stability against self-oscillation of frequencies above the 12-6Okilocycle band.

When contact 22 is on tap I'!, a switch 28 may be closed to connect acondenser 29 between the grounded shield of transformer t and terminal2|. This condenser serves to compensate for the effect of the capacityto ground associated with terminal 20 in such a Way as to make the gainchange due to a change in the setting of the GR condenser the same as itis with contact 22 on tap 20 and switch 28 open.

Condensers 31, 32, 33, 34 and 35 are stopping condensers or by-passcondensers, each of negligible reactance in the 12-60 kilocycle band.

Current for heating the filaments or heaters of the tubes is suppliedfrom battery 36, which is bypassed by a condenser 31 for reducing cross-*therefor comprising choke coil 38 and condenser 38", through resistor39 which serves to reduce cross-talk between amplifiers to which thebattery is common.

Plate and screen currents for tube P are supplied fro-m plate battery 38and filament battery 36 in series, through the resistance 39. The platecurrent passes through a choke coil 46, the coil 30 preventingalternating current of tube P from flowing through the battery 36. Thefilament heating and space current supply system for the tubes is of thegeneral type disclosed in the copending application of J. O. Edson etal.. Serial No. 82,156, filed May 27, 1936, for Amplifiers, and in theabove-mentioned Figs. 15 and 16 of the copending application of H. S.Black, Serial No. 114,390.

Grid biasing voltage for tube V1 is obtained principally from thevoltage drop across a net-' work 4! in the cathode lead of the tube,this network consisting of a biasing resistor and a by-pass condensertherefor of relatively small reactance in the band.

Grid biasing voltage for tube V2 is obtained principally from thevoltage drop across a network 42 in the cathode lead of the tube, thisnet- 7 work consisting of a resistor and a by-pass condenser therefor.

With switches 43 and 44 operated to the posi tions shown in thedrawings, grid biasing voltage for tube P is obtained from voltage dropacross a network 55 and the choke coil 48 which are in series with eachother inthe cathode lead of the tube. The network consists of aresistance shunted by a capacity and the choke coil is bypassed by thecondenser 34. The biasing voltage is filtered by a resistor 33' and bythe by-pass condenser 33. Switch 44 may be closed to shortcircuitnetwork 45 for a purpose referred to hereinafter; and then, in order tomaintain the proper grid bias, the switch 43 is operated to the left toinsert a grid biasing battery 46 in series with a resistor ll. Theresistor 47 in series with the battery 46 prevents a short circuit onthe grid lead to any of the amplifiers common to the battery fromdisabling the remainder of those amplifiers. A by-pass condenser 48across batteries 36 and serves to limit the impedance common to thebiasing circuits of several amplifiers such as the one shown and soholds crosstalk due to such coupling within tolerable limits.

A network in the cathode lead of tube V1 provides local negativefeedback around the tube, for improving the singing margin of theamplifled in the general manner explained inD. D. Robertson Patent1,994,486, March '19, 1935.

Networks 52 and 52 in the cathode lead of tube V2, which can beby-passed by a condenser 53 of negligible reactance in the transmissionband of the amplifier by closure of a switch 54, and the network 45 inthe cathode lead of tube P, which can be short-circuited by closure ofswitch 44., facilitate obtaining gain andphase conditions fortransmission around the feedback loop that avoid singing for alladjustments of the movable contact of the potentiometer l on the taps 9,l0 and l i. For a give-n setting of the condensers GC and GR, the flatgain of the amplifier is at its maximum Value when the contact is on tap9, with contact 22 on terminal 20 of the input trans-former and switches54 and 44 closed. For a gain reduction from this maximum, the movablecontactof potentiometer l is adjusted to tap IE1 and switch 54 isopened. For a further gain reduction the movable contact is adjusted totap ii, and switch 44 also is opened. For a further gain reductioncontact 22 is adjusted to tap [l and switch 2| is closed.

The suppressor grids of tubes V1 and V2 are connected to ground insteadof to the cathodes of the tubes to increase the singing margin of the.amplifier, as claimed in the copending application of F. B. Anderson,Serial No. 158,281, filed August 10, 1937, for Feedback amplifiercircuit.

The output bridge B should not only have suitable transmission andimpedance properties in the operating band of 12 to 60 kilocycles, butshould have favorable phase properties for the feedback loop above theband notwithstanding the high generator impedance in the pentode tube P.In accordance with the invention, with the constant resistance outputbridge configuration a satisfactory high frequency phase characteristicis obtained by associating with the load, including the outputtransformer T and its associated shunt capacities 6i and G2 referred tohereinafter, a network N to render the receiving impedance (i. e., theimpedance of the bridge arm containing the load) a substantiallyconstant resistive impedance over not only the operating frequency band,but also a wide frequency range above that band. In the drawings, thatbridge arm is designated K and the network N is shown connected inserial relation with the primary winding of the. transformer, betweenthe plate of tube P and that winding. The high impedance RP of thepentode P accentuates diificulties that the transformer and bridgeconstants contribute to stabilizing the feedback loop againstoscillation at frequencies above the operating frequency band. However,such difiiculties are overcome by network N, which builds thetransformer with its associated shunt capacitances El and 52 out to anetwork K which has a constant resistance impedance.

Fig. 2 shows an output bridge B which is substantially the output bridgeB with the network N omitted and with the transformer T, capacities GIand 62, and line I replaced by a substantially equivalent circuit. Inthis latter circuit the capacity 61 is represented by equivalentcapacities 6|, 66 and '61, the capacities 65 and El being capacites toground of the primary winding of the transformer andhaving values of 22to 25 mini. each; for example, the capacity 62 is represented by itsequivalent capacity 62; the line I is represented by its equivalentimpedance h; an inductance 63 represents the transformer primary leakageinductance; an inductance 65 represents the transformer secondaryleakage inductance; and an inductance 65 represents the transformermutual inductance. The network comprising elements 6|, 62', 63, 64, 65,'56 and 61 is designated T.

Where the transformer T, which may have, for example, a nominalimpedance of 3,500 ohms on the primary side and 140 ohms on thesecondary side, must have a large mutual inductance so that itsmodulation will be small, a fairly large leakage inductance results, asfor example, a leakage inductance of approximately 5.2 millihenries. Ina circuit such as that of Fig. 2, the transformer T with small inherentcapacities and relatively large leakage inductance would have itstransmission characteristic unsatisfactory in the 12-60 kilocycle band,and would have its impedance unsatisfactory both in this band andoutside of the band, especially above the band. To improve thetransmission and impedance in the band, condensers can be added on eachside of the transformer building out the inherent capacities to thecapacities 6i and 62 which may respectively have, for example, values of477.5 mmf. and 3,980 mmf, so that the equivalent structure, neglectingthe mutual inductance, is a full section of mid-shunt terminatedconstant K lowpass filter. (In the equivalent circuit as shown in Fig.2, on a 3,500 ohm basis, the capacity E2 would be 159.2 mmf., or 3,980mmf. divided by the impedance ratio 3500/ 140 of the transformer. Ofthis 159.2 mmf. capacity 62, only some mmf., for example. is transformercapacity; and similarly, of the 465 mmf. capacity 6 I only some 30 mmf.,for example, is inherent transformer capacity.) However, though thetransformer with capacities 6| and 62 has good transmission andimpedance characteristics in the 12-60 kilocycle band, its impedanceabove the band is unsatisfactory in the circuit of Fig. 2, particularlyas the capacities BI, 62, 65 and 61 produce resonances with the leakageinductance of the transformer and the inductance of the arm Z21 of theoutput bridge. These resonances may upset the performance of the outputbridge and cause great deviation from the loop gain and loop phasecharacteristics that would be obtained if the bridge arm containing theoutput transformer were a constant pure resistance. Some idea of theeffects of these resonances may be gathered from consideration of thevoltage existing between plate and cathode of the output tube at theentrance to the output bridge in response to a unit drivingelectromotive force acting on the output tube. This plate-to-cathodevoltage will depend upon the external impedance between plate andcathode, that is, the impedance attached to the plate and cathode. Thisimpedance will decrease as series resonance occurs between the low sidecapacity 62' of the transformer (and the line capacity) and the leakageinductance '63, 64. This resonance may occur in the neighborhood of 150kilocycles, for example. The resistance offered by the line l1 will dampthe resonance. The impedance increases as parallel resonance between thehigh side capacity BI of the transformer and the leakage inductance 63,M and capacity 62' in series follows at a higher frequency, (forexample, a frequency in the neighborhool of 200 kilocycles), decreasesas series resonance occurs, for example, around 300 kilocycles, between6| (together with 63, 84 and 62') and the inductance of the bridge armZ21; and rises again as parallel resonance of this inductance occurswith the capacities to ground, 66 and 6'! and others present in theequipment, for example, around 400 kilocycles. (Capacity BI is muchgreater than capacities 66 and 6'5, so that capacities 6B and 6! may beconsidered tied together, or connected in. parallel, at frequenciesabove the parallel resonance of capacity GI and the leakage inductance.)These capacities predominate at higher frequencies, and the impedancedrops as frequency increases.

The impedance-frequency characteristic of the external circuit between.the plate and the cathode thus undulates as series and parallelresonances alternate. Impedance arising with frequency is associatedwith an inductive phase angle for the transmission from the tube to thefeedback path, and the loss for such transmission decreases withfrequency increase. Similarly, decreasing impedance gives a capacitivephase angle (1. e., a negative phase angle) for transmission from tubeto feedback path, and. loss for such transmission increases withfrequency with the phase angle thus negative. The higher the internalplate resistance of the tube, the less it damps the undulations. Withthe pentode tube the oscillations may be severe, causing thetransmission and phase characteristics to deviate considerably fromthose for the case in which the output transformer and line present aconstant pure resistance to the output bridge, and resulting in gain andphase characteristics for the feedback loop that menace the margin ofthe amplifier against singing or self-oscillations of the feedback loop.

To obviate such menace, the network N shown in Figs. 1 and 3 builds outthe impedance of the transformer network or load (i. e., the impedanceof the transformer with its associated capacities 6i and 62 and line I)so that the receiving network K presents to the output bridge animpedance which is a constant resistance, excepting distributedcapacity. Network N is made in the form of a high-pass filter with aresistance termination, and is used as a two-terminal network in serieswith the primary winding of transformer T. Above 12 kilocycles thetransformer with its associated capacities GI and (i2 is equivalent to alow-pass filter (whose cut-off is above 60 kilocycles), shown as thenetwork T in Fig. 2, since the admittance of the transformer mutualinductance represented at 65 in this equivalent network T may beconsidered negligibly low above 12 kilocycles. The transformer with itsassociated capacities BI and 62 is designed to make the resistancecomponent of the impedance that this equivalent low-pass filter T withits termination l1 presents to the bridge substantially constant overthe operating frequency band of the amplifier. The high-pass filter Nadded in series with the transformer T, or lowpass filter T, issubstantially complementary to the low-pass filter T, being designed tocancel the reactance component of the impedance of the low-pass filter Tin the operating band of the amplifier and maintain the two-terminalimpedance that the combination of the two filters presents to the bridgea constant resistance on up to frequencies where parasitic bridgingcapacities take toll.

Fig. 3 shows the network N inserted in series with the network T, butotherwise has the same circuit configuration as Fig. 2. In Fig. 3 thecircuit representing the equivalent of the bridge B of Fig. 1 isdesignated B. The capacity to ground of network N at its terminalconnected to the plate of tube P is designated 68, and is considerablysmaller than capacity 56, being approximately 12 mmf., for example.

The capacity 62 across the line winding of the transformer is madesufiiciently small to insure that the series resonance of the leakageinductance and the line side capacity is highly clamped by the lineresistance so that the succeeding parallel resonance between theseelements and the capacity 6| will be well damped. These series resonantelements are then effectively shunted out of the picture by the capacity6! at frequencies above the frequency of the series resonance. Thenetwork N added in series with the output transformer holds up theimpedance of the bridge arm containing the transformer at highfrequencies, rendering the impedance substantially purely resistive tofrequencies above 1000 kilocycles. The reciprocal filter N, beingsubstantially a refrequencies above the common cute two filters, cannotseries resonate with lie capacity across the arm Z21 of the outputbridge, 1. e. with the capacity to ground, 66, 61, of the outputtransformer. A highly advantageous feature of the circuit of Fig. 1, orthe circuit of Fig. 3, with network N, as compared to the circuit ofFig. 2 with the network omitted, lies in the removal of the capacity toground 66, 61, of the output coil from the plate of the tube and thesubstitution of a constant 3,500 ohm resistance (this 3,500 ohms beingthe impedance of the bridge arm that contains network N and the lowpassfilter T). The capacity 68 is introduced at the plate, but it isconsiderably smaller than the capacities 66 and 61 and an important netreduction of capacity results. In a particular amplifier, as shown inFig. 1, this lifted 35 mmf. to ground oif the plate, reducing the highfrequency capacity approximately from mmf. to 80 mmf. and resulting insubstantial increase of gain for transmission from tube to feedback pathand substantial favorable change in phase shift forsuch transmission.The capacity to ground, 66, 61, still parallel resonates with theinductance of the arm Z21 of the bridge, and should bemade as small aspracticable, especially since an attempt to eliminate this effect byinserting the network Nbetween the transformer T and the arm Z21 has thedisadvantage that the capacity to ground of the output coil is moredetrimental when directly connected to the plate than when separatedfrom the plate by a 3,500 ohm resistance as in the case of Fig, 1. Evenin the case of Fig. 1, as can be readily seen by inspection of Fig. 3,when frequency increases to values at which the inherent capacitiesacross the bridge arms Z11 and Z21 become very low impedances, two 3,500ohm resistances, one the impedance of network N and low-pass filter Tand the other the impedance of bridge arm R, are paralleled across the3,500 ohm termination F. Thus, a total resistance of about 1,200 ohms inparallel with the capacities to ground is shunted across the output ofthe tube. This reduction in impedance level means a reduction in gainfor transmission from the tube to the termination F, and a negativephase angle for such transmission. Ordinarily, for forestalling thereduction in impedance level at the plate of the tube, locating thenetwork N at the plate terminal of the primary winding is preferable toeither locating it at the other terminal or splitting it and locating apart at each terminal. Similarly, connecting network N in series withthe low-pass filter T constituted by the transformer T and capacities 6|and 62, ordinarily is preferable to shunting across the input terminalsof the low-pass filter a highpass filter or network designed to buildout the bridge arm containing the transformer to a constant resistanceimpedance, on account of the irreducible capacity inherently bridgedacross the primary winding of the transformer.

In the amplifier of Fig. 1, the two-terminal network N connected inseries with the output transformer network T eliminates the shunting ofthe transformer capacity-to-ground across the output tube and at thesame time builds out the transformer arm of the constant resistantequalizer bridge to a substantially constant resistive impedance overthe operating frequency band of the amplifier and over a higherfrequency range, for example, twenty times the operating band. Thus, thebuilding-out network holds up the gain of thetransmission from the tubeto the feedback path at high frequencies and reduces the phase shift ofsuch transmission due to the dis-'- tributed capacity to ground on thetube plate and the associated equipment, and gives the feedback loopsmooth gain and phase characteristics suitable for stabilizing theamplifier against singing at high frequencies. At the same time, asexplained hereinafter, the building-out network causes the impedance ofthe transformer as viewed from line I, or the impedance of the amplifieras viewed from line I, to have a desirable value, as for example, avalue of ohms approaching a match of the line impedance over theoperating frequency range.

The transformer T with-its associated capacities 6| and 62 may bereferred to as the transformer network. Since this transformer networkis designed for transmission only over the operating frequency band ofthe amplifier and to offer some discrimination outside of this band, itsimpedance can be designed to approach pure resistance over only thisband. As indicated above, to build out this network to a networkpresenting a 3,500 ohm resistance to theamplifier at frequencies from 12to 1000 or more kilocycles and presenting substantially a constant 140ohm resistance to line I over the operating frequency band of 12 to 60kilocycles, the transformer network is regarded as a low-pass filterover the 12 to 1000 kilocycle range and an appropriate complementaryhigh-pass filter N is connected in series with it. A series combinationof complementary high-pass and low-pass filters can be made to have animpedance which is a constant resistance from zero to infinite frequencyat the end where the filters are connected together, as disclosed, forexample, in E. C. Norton Patent 2,076,248, April 6, 1937. Suchcombination is shown, for instance, in Fig. 2 of that patent. Fig. 1herein shows the equivalent network of transformer T with its associatedcapacities 6| and 62, on a 3,500 ohm basis. In Fig. 4, XM and RMindicate the reactance and resistance components of the mutual impedanceof the transformer; and X1. and R1. indicate the leakage reactance andresistance. From Fig. 4 it can be seen that a lowpass filter structureemploying a series inductance shunted at each end by a capacitance, asin filter l2 of Fig. 2 of the Norton patent, can absorb the equivalentnetwork T of the transformer T with its associated capacities 6| and 62,neglecting the mutual impedance, which, as indicated above, hasrelatively small admittance above 12 kilocycles.

Therefore, the network of Fig. 4 is treated as the low-pass filter of aseries combination of high-pass and low-pass filters of which thehighpass filter is the network N and the low-pass filter is designatedT", this combination being shown on a 3,500 ohm basis in Fig. 5, and theconstants for the elements of the combination network are chosen inaccordance with the teaching of the Norton patent to give a networkwhose input impedance would be a constant resistance from 0 to infinitefrequency in the ideal case; and then the input impedance of the actualnetwork, comprising the building-out network N and the transformer Twith its associated capacities 6| and 62, is very nearly a constantresistance from 12 to 1000 kilocycles, since the mutual impedance of thetransformer over this range is so large as to have little effect uponthis input impedance. In each of the filters T" and N of Fig. 5, thefirst branch, that is, the branch adjacent the resistance R0 or R0, isin shunt rather than in series as in filters l2 and I3 of Fig. 2 of theNorton the network and the load circuit as viewed from the loop suitablefor stabilizing said' system against said singing.

2. A wave translating System comprising an amplifier having a negativefeedback loop and a circuit for connection to said loop including animpedance corrective network, said circuit without said network havingits impedance outside the operating frequency range of the amplifier sodiffer from its impedance within that range as to cause objectionablesinging tendency of theloop outside that range, and said network givingthe impedance of said circuit as viewed from the loop a value thatreduces saidsinging tendency.

3. A negative feedback amplifier comprising a closed feedback loop, aload circuit for connection to said feedback loop whose impedance atfrequencies outside of the operating frequency range of the amplifierdeparts from its impedance within said range in a manner that producesobjectionable singing tendency of the amplifier at said frequenciesoutside of said range, and an impedance corrective network interposedbetween said feedback loop and-said load circuit, the attenuationproduced by said network in transmission from said feedback loop to saidload circuit having its value for frequencies of said range low relativeto its value for said frequencies outside of said range, and theimpedance of said network and said load circuit faced by said feedbackloop at said frequencies outside of said range having a value thatsubstantially reduces said singing tendency.

4. A wave amplifying system having a negative feedback loop, a loadcircuit for connection to said loop, and means for preventingself-oscillation of said loop at frequencies above the frequency rangeutilized in said load circuit, said means comprising a high-pass filternetwork having an input impedance, the resistive component of whichvaries with frequency inversely to the resistance of said load circuitand the reactive component of which is equal and of opposite sign tothat of said load circuit, and means for connecting said network betweensaid loop and said load circuit with said input impedance serving as atwo-terminal impedance in series with said load circuit.

5. A wave translating system comprising an amplifier, a feedback pathforming therewith a loop producing negative feedback in the operatingfrequency range of the amplifier, a transmission circuit attached tosaid loop including a load circuit and an impedance corrective networkinterposed between said load circuit and said loop, said transmissioncircuit without said network having impedance whose value afiordsstability against amplifier singing in said range but producesobjectionable tendency of the amplifier to sing at a frequency outsidesaid range, said network causing the impedance of said transmissioncircuit presented to said loop to afford stability of the amplifieragainst said singing outside said range, and said network having itsloss for transmission from said loop to said load circuit low for thefrequencies of said range compared to its loss for said frequencyoutside said range.

6. A wave translating system comprising wave amplifying means having anegative feedback loop, a load circuit for connection to said loop whoseimpedance outside the frequency range utilized in said load circuitproduces objectionable singing tendency of said loop, and a networkinterposed between said loop and said load circuit producing substantialchange in the value of the impedance into which said loop works intransmitting to said load circuit to reduce said singing tendency whileproducing relatively little effect upon transmission from said loop tosaid load circuit in said frequency band.

7. A wave amplifying system comprising an amplifier having a negativefeedback loop, a load circuit for connection to said loop whoseimpedance outside the frequency range utilized in said load circuitcauses singing around said loop, and a network interposed between saidloop and said load circuit for giving the impedance into which said loopworks in transmitting to said load circuit a value that prevents saidsinging while producing substantially no loss in transmission from saidloop to said load circuit in said frequency band, said networkcomprising a ladder type structure the product of whose series and shuntimpedances and corresponding impedances in said load circuit is constantindependent of frequency.

8. A wave translating system comprising an amplifier having a forwardlytransmitting path and a feedback path for providing negative feedback inthe amplifier, a transmission circuit for connection to said amplifier,a transformer circuit interposed between said amplifier and saidtransmission circuit, and means for connecting said forwardlytransmitting path to said feedback path and to said transformer circuitand rendering the impedance facing said forwardly transmitting path aconstant resistance over a frequency range that includes the operatingfrequency band of the amplifier and is a number of times as wide as saidband, said means comprising a network connected to said transformercircuit that has its reactance equal and opposite to that of saidtransformer circuit over said frequency range and has its resistancevarying inversely to that of said transformer circuit over said range.

9. A wave translating system comprising an amplifier, a feedback pathfor producing negative feedback in said amplifier, a circuit forconnection to said amplifier, a transmission equalizing bridge circuitconnecting said amplifier to said feedback path and said first-mentionedcircuit, an output transformer for said amplifier having one winding ina branch of said bridge and an inductively related winding in saidfirstmentioned circuit, and a reactive impedance device in said branchfor building out the impedance of said branch to a constant resistanceover the operating frequency band of said amplifier and over a widefrequency range above the operating frequency band of said amplifier.

10. A negative feedback amplifier having an output transformer circuitand a high-pass filter with its input end in serial relation with theprimary winding of said transformer for building out the input impedanceof said circuit to a substantially constant resistance over and abovethe operating frequency band of the amplifier, said filter having itscut-off frequency suificiently above said band to render the outputimpedance of said circuit a substantially constant resistance for thefrequencies of said band.

11. The method of controlling the phase shift of propagation around thefeedback loop of a negative feedback amplifier having an unbalancedoutput bridge circuit working into a load impedance that causesobjectionable singing tendency of the amplifier at frequencies above theoperating frequency band of the amplifier,

which comprises building out the load impedance to a substantiallyconstant resistance receiving circuit over a wide frequency range abovesaid band and at the same time maintaining transmission from saidamplifier to said load impedance substantially unaltered in said band.

12. A wave translating system comprising an amplifier, a feedback pathfor producing negative feedback in said amplifier, a circuit forconnection to said amplifier, an unbalanced bridge network connectingsaid amplifier to said feedback path and to said circuit with saidcircuit in one branch of said bridge network and said feedback path in abranch which would be conjugate at bridge balance, the unbalance of saidbridge network rendering the feedback dependent upon the impedance ofsaid circuit and said circuit having its impedance at frequencies abovethe operating frequency band of said amplifier of such value as toproduce objectionable singing tendency of said amplifier, and animpedance corrective device connected in said one branch thatsubstantially reduces said singing tendency without material deleteriouseffect upon transmission between said amplifier and said circuit in saidband.

13. Au amplifier comprising a high impedance pentode output tube, afeedback path for producing negative feedback in said amplifier ofoutput Waves from said tube, a load circuit for connection to saidamplifier, an unbalanced output bridge circuit for said amplifiercomprising said tube, said path and said load circuit, said feedbacklowering the amplifier output impedance presented to said load circuitin the operating frequency band of the amplifier and the impedance ofsaid load circuit producing objectionable singing tendency of saidamplifier at frequencies above said band, and an impedance correctivedevice connected to said load circuit for substantially reducing saidsinging tendency, the attenuation of said device for transmission fromsaid amplifier to said load circuit having its Lil value for frequenciesof said band low relative a") to its value for said frequencies abovesaid band.

FRITHIOF B. ANDERSDN. ANDREW W. CLEMENT. IRA G. WILSON.

